Just took delivery of my new Elite Spectrum 100" screen and am getting ready to install.

The instructions that came with the screen cover hanging it from a chain and screwing it to a backing wall, but they do not cover screwing it directly into the ceiling. My ceiling isn't very high, so I don't want to sacrifice any space between the ceiling and the screen case for a chain, and the screen will have speakers behind it (it's acoustically transparent), so there isn't a backing wall.

The mounting holes that are intended for chain-hanging are arranged such that I could use them to screw into the ceiling; is there any reason I couldn't use them for that?

There isn't a ceiling joist where I want to mount the screen, so I'd be using toggle bolts. Four of them, rated for 50 pounds each for ceiling use. My ceiling is 3/4" plaster. The screen weighs 20.1 pounds, so I'd be well below the rated strength of the bolts; each would have to bear a bit more than five pounds.

Is there any reason I shouldn't do this?

My original plan was to use an in-ceiling trim kit, but the WAF is extremely low on that option.

you might want to put a level on the ceiling first.. just to make sure it is level. In my 1950s house it is not...
Kind of hard to see in the picture but the screen is level and the ceiling is well off over the length of the screen.
That is fastened to the wall using 2 pieces of all thread screwed into 2 Hilty toglers with washer and nuts.. this screen is about 60lbs.. no problem
The screen will not hang properly and will wrinkle if it is not perfectly level..